This invention relates to improvements in electrographic recording. It relates more particularly to a method and means for improving electrographic images.
When information is recorded on an electrographic storage medium such as a film or fiche, an area of the medium, referred to herein as the frame area, is electrostatically charged. Then the entire frame is exposed to a light image. Those portions of the frame area which are illuminated--corresponding to the light areas on the original--lose their charge. These portions corresponding to the dark areas of the original retain their charge, so that a latent image is impressed on the frame area. Following this, toner in the form of electroscopic powder or the like is applied to the frame to make the latent image visible and the toner is then fused to the medium to make the image permanent.
When the toner is applied to the latent image on the medium, it has a tendency to migrate laterally across an evenly charged frame area on the medium towards the edges of that area. This gives rise to a so-called "frame effect" which occurs because of the inability to completely discharge the background of the medium to low voltages by exposing the medium to light. Also, the background voltage on the film terminates with a sharp transient at the edge of the medium frame where the medium contacts the "charging window" formed by a conductor at the processing station. Toner particles tend to accumulate at the site of the voltage transient, causing a general darkening of the image outline on the medium that spoils the image.
It has been proposed to avoid this problem by providing a charged border around the image area. The effect of the charged border is to minimize the residual charge on the fully exposed portions of the latent image. In some cases the charge is equal to the initial charge impressed on the medium as described in U.S. Pat. Nos. 3,140,945, and 3,671,121. In others, it is greater as described in U.S. Pat. No. 3,883,349.
The provision of such a charged border does alleviate the problem to some extent. However some general darkening of the image area still occurs and spoils image definition, particularly when the medium is charged to a high potential. Also the adverse effects become more noticeable when larger image areas are exposed.
It is believed the problem stems from the inability of the charged border to offset the effects of the high residual voltage that is present on such film with a high initial charge. In other words, when toner is applied to the film during the developing step, a large percentage of the toner particles never "see" or are attracted by the border charge and are thus captured by the high residual charge on fully exposed portions of the image area thereby darkening the background. This "visibility" problem, of course, becomes more aggrevated as the size of the image area increases.